Recently, in order to improve fuel efficiency of the automobile internal combustion engine from standpoints of global environment problem, development of a fuel direct injection engine which injects gasoline or light oil directly into a combustion chamber has been carried out aggressively. In the fuel injection pump for use in such an engine, a reciprocating system employing a cam to obtain a fuel feeding pressure is used. A combination of a cam and a cam follower (cam roller, tappet and the like) for use here is one of sliding components operated under a very severe condition. Improvement of the fatigue characteristic and the wear resistance and reduction of a wear loss are very effective means for not only maintaining the performance of the fuel pump and intensifying the durability thereof, but also for improving the efficiency of the-entire engine.
For the reason, Japanese Patent Application Laid-Open No. HEI5-340213 has disclosed a trial in which TiN coating film of R.sub.z 0.02-0.5 .mu.m in terms of surface roughness is provided on a shim sliding surface opposing a cam of an operating valve train so as to reduce wear loss of both. This surface hardening film has a thickness of about 0.5-10 .mu.m.
According to this published document, because when the shim surface coated with TiN slides relative to the cam surface, the cam surface is ground by the shim surface, then the surface does not have to be ground at a high accuracy preliminarily.
In this case, the sliding surface relative to the cam is made of chilled alloy iron and it is finished to about 3.2 .mu.m in terms of the surface roughness.
Further, Japanese Patent Application Laid-Open No.HEI6-2511 has disclosed a structure of a shim in which the surface roughness of a contact surface relative to the cam is R.sub.z 0.2-0.7 .mu.m for the same object and the same purpose with the publication described above. Japanese Patent Application Laid-Open No.HEI6-137404 has disclosed a shim composed of high hardness ceramics whose surface is finished in the same way.
The cam driving mechanism disclosed in any publication is applied to a single-mountain (one convex curved surface) type. Such a type of the fuel pump cam driving mechanism is suitably applicable to a vehicle having a sufficient capacity in engine cylinder number and space like a large-size diesel commercial vehicle and cannot be used for a vehicle having a fuel pump to which this type cannot be applied unless any modification is carried out.
There is a need for reducing an entire volume and weight of an engine in an engine which injects gasoline directly (hereinafter this direct injection may be referred to as direct injection, but this is the same meaning) for a small passenger vehicle or a fuel pump for diesel direct injection engine. Therefore, for the reciprocating mechanical parts, reduction of weight and cost by reducing the number of necessary parts and volume or the like and conversion from the rolling friction type through cam and cam roller composed of mainly steel material to sliding friction type of the cam and shim (FIG. 1) have been urgently demanded. Further, this kind of the fuel pump has no capacity for mounting a large-size, large-weight pump containing the same number of the fuel compression mechanisms as that of engine cylinders, unlike, for example, a large-size in-line fuel pump for diesel vehicle. Therefore, usually as shown in FIG. 1, a single cylinder fuel compression mechanism containing a plurality of cam mountains is constructed and fuel is pressure fed to each cylinder of the engine. In such a mechanism, a cam roller (hereinafter referred to as roller) made of metallic material has been used as mainly a cam follower. However, in this system, a tangent line between the sliding surface of the cam and the roller during sliding becomes very complicated. Therefore, it is necessary to make the roller and cam in a rolling contact without generating a large relative sliding amount in a sliding portion between the roller and cam. Therefore, the sliding surface of the cam becomes complicated. As a result, it takes long to process the cam thereby leading to increase of production cost. Particularly if the relative sliding amount is large, this leads to an abnormal wear of the cam. If the sliding surface shape of the cam is changed due to this wear, fuel injection timing, injection amount and the like change. This is fatal to the engine. On the other hand, in case of sliding between a cam having plural cam mountains and a shim, a complicated shape design for the sliding surface for smoothing the tangent lines of the cam and shim is not necessary. However, reduction of the friction between the cam and shim and suppression of the friction amount when the above-mentioned rolling friction is changed to the sliding friction have been a prominent problem. Additionally, particularly in a gasoline direct injection engine, a cam shaft for fixing the cam is fixed directly to a cam shaft of the engine in most cases. Thus, reciprocations of the same number as a frequency multiplied by the number of cam mountains occur in the reciprocating mechanism for use in the operating valve train of the engine. Therefore, tremendous improvements in the wear resistance and fatigue resistance (pitching fatigue and the like) of the shim are necessary.
In case where the cam is fixed directly to the cam shaft of the engine, the sliding portions of the plunger for pressure feeding and holder shown in FIG. 1 are assembled together at an extremely high precision to suppress fuel leak. Therefore, in case where the shim is mounted directly on the end portion of the plunger, it is necessary to adjust the assembly precision for the cam to suppress a single-side contact with the shim and make uniform the contact portion. Therefore, conventionally, in the roller mechanism and shim mechanism shown in FIG. 1, as shown in FIG. 2 the lifter and lifter shim for reciprocating the plunger are disposed as a different part. In this case, in case where the cam mounting accuracy is adjusted by a clearance between the lifter and lifter guide (same FIG. a) or in case where the cam is mounted directly onto the plunger (same FIG. b), as shown in FIG. 3 the sliding surface relative to the cam is provided with a smooth, spherical crowning shape so as to carry out this adjustment depending on the case.
In these cases, additional parts are necessary or machining for providing the sliding surface of a high hardness material for use in the shim with a smooth, spherical shape is necessary. Therefore, this may lead to increase of production cost. A method for providing with the crowning shape without such machining has been disclosed in, for example, Japanese Patent Application Laid-Open No. SH063-289306. According to this publication, a ceramic member having a smooth surface is fit to a metallic part and the ceramic member is deformed by a stress generated by that fitting work so as to provide with the crowning shape. However, according to this method, an additional part for that fitting is necessary or it is difficult to obtain a high accuracy crowning shape.
The above-mentioned problem is noticed in only a combination of a shim and a cam having a plurality of cam mountains and therefore, no noticeable problem has been produced in an ordinary single-mountain cam. That is, although the single-mountain cam mechanism is used in the operating valve system and the like of the gasoline engine, by using a conventional steel made shim, no noticeable problem exists regarding its wear resistance and durability.
As regards the friction work by sliding, considering a use condition under an existence of lubricating material like lubricant, generally a minimum gap between opposing sliding parts, minimum oil film thickness and characteristic of the sliding surface of the sliding part largely affect the sliding characteristic and frictional loss. Generally, the friction work in the above-described sliding is expressed by a following formula. EQU F=A{.alpha.Sm+(1-.alpha.)St} (Formula 1)
Where F is friction work, A is sliding area, .alpha. is breaking area ratio of oil film, Sm is shearing strength in a case where an opposing sliding member is in a solid contact, St is shearing strength of oil film and .alpha.Sm is friction work of a case where no oil film exists (friction work under boundary lubrication) and (1-.alpha.)St is friction work of a case where oil film exists (friction work under a complete fluid lubrication). Because Sm is larger than St, it is necessary to increase the item of the friction work under a complete fluid lubrication to reduce the friction work F and that is, it is necessary to reduce .alpha..
On the other hand, it is important to control the characteristic of the sliding surface of the opposing sliding part in order to provide the sliding portion with this complete fluid lubrication. For example, in column number (0004) of Japanese Patent Application Laid-Open No.HEI7-98052, an oil film parameter indicating a scale of lubrication is indicated in the following formula (2), this formula indicates that increasing this value is effective for provision of the complete fluid lubricating condition. Further, the publication disclose that at the same time, decreasing the surface roughness of two opposing sliding surfaces is also effective for decreasing the friction work. EQU .LAMBDA.=hmin/ (Rrms1 2+Rrms2 2) (Formula 2)
Where hmin is a minimum gap between the opposing sliding parts or a minimum oil film thickness, Rrms1 is square average roughness of a sliding part surface and Rrms2 is the square average roughness of the other sliding part surface.